An integrated circuit contains various semiconductor devices and a plurality of conducting metal paths that provide electrical power to the semiconductor devices and allow these semiconductor devices to share and exchange information. Within an integrated circuit, metal layers are stacked on top of one another using intermetal or interlayer dielectric layers that insulate the metal layers from each other. Normally, each metal layer must form an electrical contact to at least one additional metal layer. Such electrical contact is achieved by etching a hole (i.e., a via) in the interlayer dielectric that separates the metal layers, and filling the resulting via with a metal to create an interconnect. Metal layers typically occupy etched pathways in the interlayer dielectric. A “via” normally refers to any micro-feature such as a hole, line or other similar feature formed within a dielectric layer that provides an electrical connection through the dielectric layer to a conductive layer underlying the dielectric layer. Similarly, metal layers connecting two or more vias are normally referred to as trenches.
A long-recognized objective in the constant advancement of integrated circuit (IC) technology is the scaling down of IC dimensions. Such scale down of IC dimensions reduces area capacitance and is critical to obtaining higher speed performance of ICs. Moreover, reducing the area of an IC die leads to higher yield in IC fabrication. These advances are driving forces to constantly scale down IC dimensions. An increase in device performance is normally accompanied by a decrease in device area or an increase in device density. An increase in device density requires a decrease in via dimensions used to form interconnects, including a larger aspect ratio (i.e., depth to width ratio). As the minimum feature dimensions on patterned substrates (wafers) steadily decreases, several consequences of this downward scaling are becoming apparent. As the width of metal lines is scaled down to smaller submicron and even nanometer dimensions, electromigration failure, which may lead to open and extruded metal lines, is now a well-recognized problem. Moreover, as dimensions of metal lines further decrease, metal line resistance increases substantially, and this increase in line resistance may adversely affect circuit performance.
The introduction of copper (Cu) metal into multilayer metallization schemes for manufacturing integrated circuits is enabled by the damascene Cu plating process and is now extensively used by manufacturers of advanced microprocessors and application-specific circuits. However, Cu cannot be put in direct contact with dielectric materials since Cu has poor adhesion to the dielectric materials and Cu is known to easily diffuse into common integrated circuit materials such as silicon and dielectric materials where Cu is a mid-bandgap impurity. Furthermore, oxygen can diffuse from an oxygen-containing dielectric material into Cu, thereby decreasing the electrical conductivity of the Cu metal. Therefore, a diffusion barrier material is formed onto dielectric materials and other materials in the integrated circuits to surround the Cu and prevent diffusion of the Cu into the integrated circuit materials.
Cu plating onto interconnect structures usually requires a nucleation or seed layer that is deposited onto the diffusion barrier. The seed layer is preferably conformally deposited over the interconnect structure prior to Cu plating. As the line width of interconnect structures is continually decreased, the thickness of the diffusion barrier and seed material needs to be reduced to minimize the volume of the diffusion barrier material within an interconnect feature containing the Cu metal fill. Minimizing the volume of the diffusion barrier material in turn maximizes the volume of the Cu metal fill. As is known to one of ordinary skill in the art, diffusion barrier materials generally have higher electrical resistance than the Cu metal fill. Therefore, maximizing the volume of the Cu metal fill and minimizing the volume of the diffusion barrier material results in minimizing the electrical resistance of the interconnect structure.
A TaN/Ta bilayer is commonly used as a diffusion barrier/adhesion layer for Cu metallization since the TaN barrier layer adheres well to oxides and provides a good barrier to Cu diffusion and the Ta adhesion layer wets well to both TaN onto which it is formed and to the Cu metal formed over it. However, Ta is normally deposited by sputtering or plasma processing methods, which are unable to provide conformal coverage over high aspect ratio micro-features. Ruthenium (Ru) has been suggested as a diffusion barrier since it can be conformally deposited, adheres well to Cu, and can serve as a seed layer. However, Ru shows poor adhesion to oxides and thus needs to be integrated with other materials for successful implementation into Cu metallization applications.